Isolated switching regulators are commonly used in consumer devices with high input voltages to convert them to lower voltages required by subsystems and safely separate users from hazardous high voltages. The isolation is usually provided by using transformer in power stage and opto-coupler in feedback path as shown in FIG. 1 for an example of an isolated flyback topology. The circuit is therefore divided to primary and secondary sides. While primary side contains gate drive, PWM comparator, and over current protection circuits, secondary-side circuit contains error amplifier and reference voltage. However, because of isolation required, secondary and primary side cannot be integrated in a single integrated circuit (IC).
Traditionally, a discrete solution such as an adjustable shunt regulator circuit (TL431 family) is used to function as the secondary-side controller (i.e., a bandgap and an error amplifier). FIG. 2 shows how a shunt regulator circuit is typically used as the secondary side controller in an isolated converter such as an isolated flyback. In this circuit, resistors R1 and R2 and capacitor Cc are loop compensation elements and resistor R3 is used to set the output voltage (i.e., Vout=(1+R1/R3)VBG, where VBG is 1.24V bandgap voltage). The primary side usually is implemented as a current-mode PWM converter, and its basic block diagram is shown in FIG. 2. To simplify the compensation and increase its robustness, in addition to current- and voltage-loops, a feed-forward path is added through opto-coupler input diode anode. More detailed information on compensation and secondary-side control with shunt regulator circuit devices is shown in FIG. 3. Although illustrated as a flyback converter, these shunt regulator circuits can be used for other isolated topologies such as a forward converters.
Discrete solutions such as shown, however, can not provide closed loop soft start to tightly control output voltage Vout during startup sequence. Available solutions with shunt regulators usually use open loop soft-start schemes that control some other voltage or current during the startup such as clamping the Vopto cathode voltage (Vopto) at startup or limiting the transformer current at primary IC. However, since these techniques control Vout indirectly (e.g., open loop), Vout may experience overshoot during startup (i.e., non monolithic startup). The problem worsens for lower Vout values. The cathode voltage of shunt regulator (Vopto in FIG. 2) needs minimum of 1.24V for operation, assuming 1V drop for opto coupler diode, and 0.75V on bias resistor Ropto1, TL431 starts taking over the loop control only when Vout has reached about 3V. This can cause soft-start problems if Vout final value is set at 3.3V.
It is also known to use an open-loop soft-start circuit using discrete components, as shown in FIG. 4. This circuit utilizes resistor R13, capacitor C18 and diode D9 to implement the open-loop soft-start circuit. Note that this circuit does not compare the output voltage with a reference during start up, so that closed-loop control is not possible.